Expectation Value-pCCD-Based Methods for Single-Electron Properties.

Expectation-value-coupled cluster theory (XCC) offers a simple avenue for molecular property evaluation. However, its potential has not been fully explored for the new computationally inexpensive CC models, such as pair-coupled cluster doubles (pCCD) and post-pCCD extensions. To that end, we implemented and explored one-electron reduced density matrices in the explicitly connected commutator expansion of the expectation value framework [J. Chem. Phys. 2006, 125, 184109] using pCCD, frozen pair Coupled Cluster (fpCC), and frozen pair linearized Coupled Cluster (fpLCC) variants. The expectation-value-based density matrices are calculated directly using the cluster amplitudes and are computationally cheaper than the corresponding response CC densities, as we bypass solving the computationally expensive Λ-equations. The performance of this approach, when combined with the pCCD-based methods, is assessed against the dipole and quadrupole moments of molecules of a varying chemical nature. We benchmarked our results against the response of CCSD(T) using Hartree-Fock canonical orbitals and variationally optimized pCCD orbitals. Our study highlights that localized pCCD orbitals are a good choice for computing one-electron properties of organic molecules.